One of the general methods for measuring an analyte in a specimen is immunoassay using antibodies for analyte (antigen) in the field of clinical chemistry. Particularly, immunoagglutination methods and immunochromatographic methods using antibodies immobilized on the insoluble carriers have the advantage in sensitivity and usability, so they are universal measurement methods applied to various clinical examination items.
Due to the recent improvements in measurement accuracy, to avoid nonspecific reactions in immunoassays is a problem in the development of in vitro diagnostic reagents. Nonspecific reaction is a phenomenon in which some kind of factor in a specimen reacts with an antibody that is used as a constituent component of the reagent employing as a principle an immunoassay and generates a false signal, which results in interference with correct measurement. The heterophile antibody and rheumatoid factor (RF) were found to be a causal factor of nonspecific reaction.
Heterophile antibody is a collective term for human antibodies exhibiting reactivity to an animal-derived antibody that is a main component for making up an immunoassay, and HAMA (human anti-mouse antibody) is known as a representative antibody. Since it is suggested that heterophile antibody may be produced by antigen sensitization under unconscious situations such as diet, contact with an animal, and administration of a biological drug and that an antibody in a sample from a human unexposed to an antigen can exhibits heterophilicity, the cause thereof is currently not clarified in detail or in a unified way. On the other hand, since rheumatoid factor appears in rheumatoid arthritis patients and is known as recognizing the Fc region of immunoglobulins (antibodies), the concept of rheumatoid factor is considered to be different from that of heterophile antibody; however, both have a common characteristic in terms of reactivity to animal-derived antibodies and it is known that both are actually human IgG or IgM (so-called autoantibody) as described in Non-Patent Document 1.
In a conventional method, antibody fragments (defined later), Fab or F(ab′)2, acquired by removing Fe region, are used as antibodies used for in vitro diagnostic reagent to reduce nonspecific reaction, and such method is particularly effective in the suppression of the effects of rheumatoid factor. Additionally, a heterophilic blocking reagent HBR, which contains as a component anti-human IgM monoclonal antibodies described in Non-Patent Documents 2 and 3, is commercialized by SCANTIBODIES as an additive agent suppressing nonspecific reaction due to heterophile antibody binding to Fab. Other methods have also been devised, including a method described in Patent Document 1 comprising adding a polyclonal antibody to IgM class natural antibody prepared from an animal of the same species as that of the antibody used for measurement, a method described in Patent Document 2 comprising adding various animal antibodies to recognition regions of rheumatoid factor, and a method of suppressing nonspecific reaction described in Patent Document 3 comprising using a single kind of anti-human IgM monoclonal antibody capable of agglutinating human IgM.
However, nonspecific reaction cannot sufficiently be suppressed only by the conventional countermeasures and it has been found out that the countermeasures themselves cause nonspecific reaction in some specimens.